The present invention relates to a miniature mercury switch, and more particularly to a position insensitive mercury switch employing an elongated armature which is movable between first and second positions in an enclosed housing.
Mercury relay switches of the type to which the invention pertains generally include a hollow, cylindrical electrically conducting component which forms the common contact of the switch. A stationary contact is disposed at each end of the common contact and insulated therefrom. The stationary contacts are conventionally supported in an insulating sleeve which is hermetically sealed to the common contact to form a sealed enclosure within the hollow portion of the common contact. A movable elongated armature, or shuttle, is located within this enclosure. The armature can be moved longitudinally from one end of the enclosure to the other by means of an electromagnetic oil assembly which surrounds the common contact. Depending upon the position of the armature, electrical contact is established between the common contact and one of the stationary contacts by means of the armature.
A thin film of liquid mercury is placed within the enclosure to ensure good electrical contact between the armature, the common contact, and the stationary contacts. For proper operation of the mercury switch, the surfaces of the stationary contacts, common contact and armature should be mercury wettable to provide the proper electrical contact. However, to avoid mercury shorting of the stationary contacts and the common contact, the material which insulates them from one another should be non-mercury wettable. Glass has been found to be an insulator which is suitable for this purpose. Mercury relay switches of this type are disclosed, for example, in commonly assigned U.S. Pat. Nos. 3,144,533, 3,786,217 and 3,867,603. The present invention constitutes an improvement upon the relay switches disclosed in those patents.
Mercury relay switches of the type described provide a miniature switch having numerous applications in a variety of DC and RF environments. These switches are capable of performing over 100,000,000 switching cycles before replacement of the switch is necessitated. One reason contributing to the need to replace the mercury relay switch is the increase in time it takes the armature to move from operative contact with one stationary contact to the other in response to an electromagnetic command signal after many cycles of use. In addition, the armature may fail to respond to a command signal after it has been idle for a long period of time.
A platinum-iridium alloy is commonly used to make the common contact, since this is a material which is mercury wettable, has a suitable coefficient of expansion for glass sealing and is nonmagnetic. One property of this alloy is its ability to combine with mercury to form a platinum-mercury amalgam. This amalgam thickens and increases viscosity with the passage of time and forms a very thick liquid at the interface of the common contact and the mercury film.
The two predominant forces acting upon the armature during the operation of the mercury switch are the surface tension of the mercury film between the armature and the common contact, and the magnetic forces produced by the biasing magnets. These forces can pivot the armature and pull it close to the surface of the common contact, particularly following long periods of storage or non-use of the mercury switch. If the armature comes too close to the common contact, it will become embedded in the platinum-mercury amalgam and be unable to move within the interior of the switch.
The formation of the platinum-mercury amalgam can be avoided by using different alloys to form the common contact. However, the wetting characteristics of other alloys which can be used to form the common contact are generally not as good as those of platinum alloys.
Furthermore, the viscosity of mercury is inversely proportional to the thickness of the film between the common contact and the movable armature. If the armature is drawn closer to the common contact, such as during non-use periods of the switch, the viscosity of the mercury film increases substantially to a point where it inhibits movement of the armature. The ability of the armature to move rapidly from one stationary contact to the other is thereby reduced on the first few operations after substantial non-use. Only after a few switching operations will the viscosity of the mercury film be restored to its initial value to thereby enable relatively rapid movement of the armature to take place.
It would be advantageous to be able to overcome the effects of these properties of mercury on the operation of the switch, and it is therefore a general object of the present invention to provide a novel mercury switch having an increased shelf life.
It is another object of the present invention to provide a novel mercury relay switch in which the movable armature is maintained in a spaced relationship from the common contact to thereby keep the viscosity of the mercury film low.
It is a further object of the present invention to provide a novel mercury relay switch in which readily available alloys can be used to form the common contact without having the armature become embedded in an amalgam at the interface of the common contact and mercury film after prolonged non-use of the switch.
Due to the nature of the materials used to provide proper functioning of the prior art mercury relay switches, the fabrication of the switches requires a highly skilled operation. Among the criteria for proper operation of the switch, the common contact must be non-magnetic to avoid adverse influence on the movement of the armature, mercury wettable to provide a good electrical contact with the armature, and sealable to glass to provide a hermetically sealed enclosure which inhibits leaking of air into the switch. In addition, it is desirable to use a material for the common contact which has a coefficient of expansion close to that of the glass which forms the insulator material in the relay switch, to avoid imparting stress to the glass-common contact seal during manufacture or under changing temperature conditions. As discussed previously, platinum has been found to be the most suitable material for the common contact, since it is easily mercury wettable, is non-magnetic, and has a coefficient of expansion sufficiently close to certain glasses which are used as insulators in the mercury switches. However, platinum will not adhere to glass in a manner which forms a suitable hermetic seal. In order to form a metal-glass seal, it is a common practice to oxidize the metal, since this procedure allows a chemical bond to be formed between the sealing metal and glass, which is a solution of metallic oxides. However, platinum is not suitably oxidizable and therefore not susceptible to being sealed to the glass through this conventional technique.
Therefore, in order to effect a hermetic seal between the glass insulator and the platinum common contact, an oxidizable metal is first plated and then diffused into the common contact. During this diffusion process, it is necessary to insure that the oxidizable metal is diffused only into the outside surface of the common contact. If the oxidizable metal should penetrate to the inner surface of the contact, the platinum will no longer be mercury wettable and therefore will not provide a satisfactory film of mercury for the switch operation. In order to render the platinum common contact oxidizable and maintain its interior surface mercury wettable during manufacture of the mercury relay switch, the ends of the common contact are capped with rubber pressure plates prior to plating. The common contact is then nickel plated and heated in a hydrogen atmosphere for a period of time sufficient to diffuse the nickel into the exterior surface of the common contact. If the diffusing process is carried on for too long a period of time, the nickel will diffuse through the thickness of the common contact and be present at the interior surface of the contact. Therefore, it is necessary to precisely control the equilibrium condition of the diffusing process. After the nickel has been diffused into the exterior surface of the common contact, it is oxidized to enable the contact to be sealed to the glass insulator.
It is desirable to reduce the number of skilled operations required during the manufacture of the mercury switch, to reduce the time and cost of production. However, the previously noted criteria relating to the properties of the common contact must be met.
It is therefore another object of the present invention to provide a novel position insensitive mercury relay switch in which the number of highly skilled operations required to produce the switch is reduced.
It is yet a further object of the present invention to provide a novel common contact assembly for a mercury relay switch, which contact assembly does not require special oxidizing procedures to enable the contact to be sealed to the glass insulator.
It is still another object of the present invention to provide a novel position insensitive mercury relay switch which is less expensive and easier to produce than prior art mercury relay switches.
As discussed previously, the position of the armature within the sealed enclosure of the switch is controlled by means of an electromagnetic control coil. The strength of the magnetic field produced by the coil, and hence the amount of force necessary to shift the armature, is a function of the magnetic reluctance of the path through which the magnetic flux flows. For a coil having a limited size and a fixed number of coil turns, the magnetomotive force produced by the coil is proportional to the power supplied to the coil. If the operating force requirements of a switch are relatively high, it becomes necessary to provide power to the coil by means of a solid state device, such as a coil driver, in order to operate the switch in response to the output signals from a transistor-transistor-logic (TTL) control circuit. This necessity is due to the fact that the output signals from the logic circuit are not capable of directly driving the electromagnetic coils of the switch.
It is therefore desirable to reduce the magnetic reluctance within the switch to thereby reduce the amount of force required by the switch for operation. This will allow the switch to be directly driven by the output signals from TTL logic circuits, further decreasing the cost of the switch in many applications by eliminating the need for coil drivers.
In addition to the electromagnetic coil, the magnetic circuit of the mercury relay switch can also employ an external permanent magnet. The permanent magnet provides a magnetic field which maintains the armature in operative contact with the selected one of the stationary contacts, to thereby render the switch position insensitive. The permanent magnet also functions to magnetically bias the armature, so that the armature will be shifted in the proper direction when the electromagnetic coil is energized.
Due to possible interference from the magnetic field produced by the external permanent magnet, the applicability of the mercury relay switch is limited in certain radio frequency (RF) environments. Furthermore, the external magnet reduces the amount of space available for the electromagnetic coils. In view of the limited coil size, the applicability of the mercury relay switch in certain low power DC environments is also limited.
It is therefore another object of the present invention to provide a novel mercury relay switch having a magnetic circuit with increased efficiency, to thereby reduce the amount of power necessary to operate the switch.
It is still a further object of the present invention to provide a novel common contact assembly for a mercury relay switch which reduces the reluctance in the magnetic circuit of the switch.
It is yet another object of the present invention to eliminate the external permanent magnet in a mercury relay switch and thereby increase the number of applications of the switch.
These, as well as other objects and advantages of the present invention, will become apparent to one of ordinary skill in the art to which the present invention pertains upon a perusal of the following description when taken in conjunction with the accompanying drawings.